The present invention relates to a method and apparatus for detecting the unbalance condition of a load of material in an appliance and more particularly, for detecting an unbalance condition of a load of material in a rotatable vessel of the appliance.
Various appliances, such as automatic washing machines, automatic dryers, centrifugal liquid extractors, etc., utilize a rotating tub, basket or other vessel holding a load of material which may or may not be evenly distributed within the vessel. The condition of having the load unevenly distributed, or out of balance, creates a situation where the center of mass of the rotating vessel does not correspond to the geometric axis of the vessel. This leads to the generation of high loads and severe vibration of the vessel. In an appliance, this severe vibration may cause the phenomenon of movement of the appliance across the floor or other supporting surface. This can occur both in vertical axis rotating vessels as well as horizontal axis vessels and also in those appliances where the axis is arranged inbetween vertical and horizontal.
Various attempts have been provided in the prior art to provide mechanical arrangements to limit or reduce the possibility of unbalanced loads, which typically involve the addition of various masses, either fixed or movable, to the vessel which requires additional power for the motor to rotate the vessel.
Approaches have also been disclosed in the prior art for detecting a load imbalance, for example, in an inverter driven motor for a washing machine, as disclosed in U.S. Pat. No. 5,070,565. That patent discloses to examine a ripple in the dc-inverter bus current, with a ripple value above a pre-determined level being indicative of load unbalance. If a load unbalance is detected, the washer controller would resume a redistribution cycle to attempt to re-balance the clothes. This would be attempted a predetermined number of times and, if the load is still unbalanced, the spin cycle would be aborted. If the ripple value falls below the pre-determined level before the maximum number of tries is reached, the spin cycle is started. Once a spin cycle has been initiated, the length of the spin cycle is determined on the basis of the magnitude of any remaining load unbalance. Spin rate and spin time may be adjusted based upon the degree of load unbalance detected.
It would be an advance if a method and apparatus were provided in which the potential for a severe unbalance could be predicted in advance of it actually occurring so that appropriate steps could be taken to avoid the detrimental effects of such a condition.
The present invention provides a method and apparatus for detecting a severe unbalance condition in a rotating device such as a basket, tub or other rotatable vessel of an appliance, for example an automatic washer. The method and apparatus provide the detecting by monitoring the motor current signature. When a severe unbalance condition is detected at a high rotational speed, the spin speed can be adaptively dropped down to a safe level, in which the appliance vibrations and mechanical stresses are tolerable. The appliance system can be continuously monitored so that, in the case of an appliance such as an automatic washer, clothes dryer or centrifugal water extraction, as water is extracted from the fabric load, and the load becomes less massive, the spin speed can be gradually increased to a desired level. If too great of an unbalance condition still persists, the spin speed can be adaptably limited or the cycle can be terminated and the user can be advised.
The effect of unbalanced loads in a motor driven rotating component, such as a rotatable vessel, translates into motor torque oscillations, which are proportional to the motor stator currents. Moreover, increased vibrations in certain appliances cause energy dissipation in passive components, such as in the suspension system, causing the average motor current to increase. In the case of a controlled induction motor (CIM), the stator currents are estimated by directly measuring the dc bus current of the inverter.
In the present invention, motor torque oscillations are monitored as the vessel rotational speed is increased in a series of steps and a severe unbalance condition is detected as soon as it happens.
A stepped speed profile is commanded to the motor by the control system in order to obtain information about the load. The average energy that is required by the appliance to spin the vessel is monitored by means of averaging the motor stator current. Under normal conditions, the motor current can increase due to several conditions: acceleration torque due to speed profiles, mechanical drag in the system (i.e., bearings friction, viscous forces, etc.) or energy dissipation by the shock absorbers in the suspension system (i.e., when severe unbalanced loads are present). By carefully choosing the acceleration rates of the speed profiles and using self-referencing techniques, it is possible to minimize the effects of inertia and mechanical drag in the system. It is therefore possible to relate the motor average current to the mechanical dissipation of the shock absorbers and to the unbalance in the load. In a typical speed profile for a spin cycle, several regions can be identified, each characterized by a speed ramp during which the speed is increased, and a plateau where the speed is held constant. The speed ramps should be carefully chosen in order to avoid masking the effect of unbalanced loads on the motor current with the effect of acceleration torque and inertia (i.e., load size). At the beginning of each of these regions, thresholds are dynamically established for the motor current. When the motor current exceeds these thresholds, a severe unbalance condition for the load is assumed. In order to minimize the effect of mechanical drag, a base line for the motor current is acquired at the beginning of each region (end of previous speed plateau). In this condition there is no contribution due to the inertia (speed is constant) and the base line reflects the mechanical drag present in the system. The threshold for the motor current is established as a fixed offset (determined experimentally) above the base line. In order to increase the accuracy of the algorithm, the offset is higher during the speed ramp (to account for higher currents due to acceleration torque) and lower during the speed plateau (no acceleration, only mechanical drag). It is then possible to dynamically create a threshold profile that matches closely the speed profile and that accounts for the system mechanical drag (base line). The thresholds are determined separately for the ramp and for the plateau portions of the spin speed profile.
To further improve the reliability of the algorithm, the thresholds are also adjusted to account for variations in line voltage. The motor current for a given torque-speed operating point depends on the voltage supplied to the motor. When driving the motor with an inverter, the maximum possible voltage is delivered to the motor during parts of the spin speed profile. This maximum voltage applied to the motor depends directly on the line supply voltage provided to the inverter and no regulation for the voltage supplied to the motor is possible in this condition. It has been experimentally determined that when the voltage is low, the current is higher for a given torque-speed point and vice versa. A simple inverse proportional compensation is used to adjust the thresholds to the line voltage.